Pi-histidine factor arose as an autonomously replicating duplication of part of the histidine region of a Salmonella typhimurium mutant. Pi was selected for its ability to express the undamaged histidine genes which its parent, hisG203 did not. The proposed research seeks to understand the origin of pi and the mechanism of its replication. To achieve this, I plan to examine the physical structure of pi-histidine DNA to see whether it is circular. I will perform a genetic analysis of pi integration and excision from the chromosome. In addition, mutants of pi deficient in replication will be isolated and examined genetically and physiologically.